1. Field of the Invention
The present invention relates to the method for mounting the ACF(Anisotropic Conductive Film) for FPC and COG(Chip On Glass), at the same time. Especially, the present invention relates to the method for reducing the manufacturing steps for mounting driver ICs using ACF in COG structure.
2. Description of the Background Art
The CRT(Cathode Ray Tube), the most general display device, reproduce the color display information using electron guns for a red color, a green color and a blue color. The thickness of the CRT must be thicker and thicker when the screen size is becoming lager and larger. Because the CRT device can reproduce the images only if the distance between the electron guns and the screen of the CRT is enough. So, the CRT device is no more a proper device when the small private display device, like TV, is replace with the large public device, like a beam projector.
In recent year, many flat display devices alternating the CRT are developed. Among them, a liquid crystal display(LCD) device has become more popular than others. Conventionally, the LCD comprises, as shown in FIG. 1, a controller IC 13, a scan line driver IC 11, a data line driver IC 10 and an array of thin film transistors(or TFT array) 16. A plurality of scan lines 15 is connected to the out line of the scan line driver IC 11, and a plurality of data lines 14 is connected to the out line of the data line driver IC 10. At the intersection area of the scan lines 15 and the data lines 14, thin film transistors 16 connecting with pixels 17 are arrayed. A gate electrode of the thin film transistor 16 is connected to the scan line 15 and a source electrode is connected to the data line 14 and a drain electrode is connected to the pixel electrode 17. When certain voltage is applied to the gate electrode of the TFT 16, then the source electrode of the TFT 16 and the drain electrode are electrically connected. And if there is no voltage at the gate electrode the source and the drain electrodes are electrically isolated.
The conventional method for reproduce the image information in the LCD device is like below. The image information is converted into a data voltage at the controller IC 13 and the data voltage is held at the data line driver IC 10. The data line driver IC 10 put out the data voltage to the data line 14 according to the scan signal. For example, when the scan line driver IC 11 put out the scan voltage at the first scan line 15 according to the predetermined frequency signal, the TFTs 16 connected to the first scan line 15 are turn on and the data voltages of the first line of the image information are applied to the first line of the pixel 17 electrode array. And then, when the scan line driver IC 11 put out the scan voltage at the second scan line 15, the data line driver IC 10 put out the second line of the image information are applied to the second line of the pixel 17 electrode array. With the same method as mentioned above, the other lines of the image information applied to the other lines of the pixel 17 electrode array. So, the image information is reproduced at the screen of the LCD.
Referring to cross sectional view of the LCD shown as in the FIG. 2, the LCD comprises a lower plate 21 having the TFT 16 arrays and a higher plate 20 having a common electrode 23 and a color filter 22. Generally, the lower plate 21 is larger than the higher plate 20. The reason is that the lower plate 21 should have a second space 26 for mounting driver ICs(27, 28) and for FPC connecting to the driver Ics(27, 28), except the first space 25 for the TFT 16 arrays.
Generally, when the driver IC or the FPC is mounted to certain device, an anisotropic conductive film(ACF) coating an anisotropic conductive adhesive on a protective film is used. Referring to the FIG. 3, the ACF have the structure in which the conductive balls are scattered in the adhesive resin 30. The conductive ball comprises a conductive sphere 32 covered with a thin insulation membrane 31. The insulation membrane 31 can be easily broken under pressure. So, when two electrodes should be electrically connected, the ACF inserted between the two electrode and pressed. Then the insulation membrane 31 is broken and the conductive sphere 32 is connected with the two electrode.
There is an tape automated bonding(TAB) method for connecting the driver IC and terminal pad of the TFT arrays. The TAB method comprises an inner lead bonding(ILB) step and an outer lead bonding(OLB) step in which a film having conductive bus lines are connected with electrodes of the driver IC. The ILB step is to connect the electrode of driver IC to one side of a film lead with a bump, and the OLB step is to connect the terminal pad to the other side of the film lead. Referring to the FIG. 4, the TAB method is explained more detail. The bumps 41 are formed on the electrode 40 of the driver IC() (FIG. 4a). One side of a polyimide film 43 having conductive bus lines is mounted on the bumps 41(FIG. 4b). The bumps 41 and the film 43 are bonded by pressing with high temperature. Additionally, a protective resin 44 is coated on the connected part in order to enhance the resist against the moisture or stress (FIG. 4c). After that, the another side of the polyimide film is connected to the terminal pad of the LCD with an ACF using the similar process. Here, the conductive balls embedding in the polyimide film are connected to the lead line and the terminal pad. Consequently, in the TAB method, the driver IC is mounted out side of the LCD panel and the driver IC is connected to the terminal of the LCD using a film having conductive bus lines.
Another method is a chip on glass(COG) method in which the driver ICs are manufactured on the glass of the LCD panel. In the COG method, the driver ICs are connected to the terminal pad with the bumps and the ACF film instead of using the polyimide film having conductive bus lines in TAB method. Therefore, the manufacturing process and the structure of the LCD is more simple. Furthermore, the screen area is larger in the same size of the glass panel.
Referring to the FIG. 5, the mounting method for driver ICs by the COG is explained. On the terminal pad 52 formed on the glass 50 of the LCD panel, an anisotropic conductive film(ACF) 30 having conductive balls 51 is mounted (FIG. 5a). A driver IC having bumps 53 on the electrode is arranged on the ACF 30. And the driver IC and the terminal pad are pressed to adhere each other. So, the conductive balls 51 are connect the terminal pad and the electrode of the IC as the insulation membrane covering the conductive ball 51 are broken. Finally, high temperature(over 120 centigrade) is applied to the ACF to harden it, so the driver IC is adhered on the glass safely.
Referring to FIG. 6, the method for mounting the ACF on the terminal pad or electrode is explained. An ACF 61 having the similar size with the terminals or electrode wanted to mounting is weakly adhered on a protection film 60(FIG. 6a). The ACF 61 is mounted on the terminal 62 with alignment(FIG. 6b). Here, the protection film 60 is protects the ACF 61 from the pollution materials. On the protection film 60, a pressure is applied with a temperature(about 100 centigrade). Then, the protection film 60 is depart from the ACF and the ACF adheres on the terminal 62. Because the adhesion between the ACF and the terminal is more strong than that of the ACF and the protection film.
Referring to the mentioned above, the manufacturing method for the lower plate of the LCD will be explained as shown FIGS. 7 and 8. On the substrate 70, a gate electrode 71 and a scan line(not shown) are formed using a metal(FIG. 7). On the gate electrode and the scan line, a first insulation layer 72 is deposited(FIG. 7b). A source electrode 74, a drain electrode 73 and a data line are formed on the first insulation layer 72 using a metal(FIG. 7c). A doped semiconductor layer 75 and a intrinsic semiconductor layer 76 are formed sequently. After that, a second insulation layer 77 is deposited and a contact hole for exposing the drain electrode 73(FIG. 7d). A pixel electrode 78 connecting to the drain electrode 73 is formed on the second insulation layer 77(FIG. 7e).
At the first area 25 of the substrate 70, the TFT arrays mentioned above are formed. And at the boundary area of the first area and the second area, the terminals of the scan lines and data lines are formed. At the second area 26 of the substrate 70, there are two area. One area is chip boding area 81 in which the driver ICs are mounted. The other area is FPC bonding area 82 in which FPCs applied input data from the video controller are mounted. At the boundary area and the chip bonding area, a first ACF for COG is adhered. At the FCP bonding area, a second ACF of FPC is adhered. After that, a driver IC 83 is mounted on the first ACF to connect to the terminals of scan and data lines, and a FPC 84 is mounted on the second ACF.
Generally, the ACFs for COG and FPC are made and supplied in different ways. So, in order to adhere the ACFs for COG and FPC, two steps are needed. However, the ACFs for COG and FPC are similar materials, so the processing for adhere the ACFs is duplicated without any meaning. Furthermore, tolerances are occurred when the two types ACF are mounted in separated processes. So, the quality of the LCD is hindered. And the manufacturing tools and tact time are wasted. Especially, in the large LCD panel such as over 12 inch, it is necessary to reduce the alignment tolerance for connecting the FPC and the driver IC, because the number of driver IC is large and the length of the ACF is long. Additionally, the distance between the ACFs for COG and FPC is so short that the hardening process of one ACF should give unpredicted results to the other ACF.